🧪 Add tests for pic_contracts.diff_cli.main

[edithatogo]

🎯 What: Function main in pic_contracts/diff_cli.py lacked test coverage.
📊 Coverage: Added tests verifying successful CLI execution handling Markdown and JSON outputs, and test error handling (SystemExit) for missing files or missing arguments.
✨ Result: Reached 100% test coverage for pic_contracts.diff_cli.

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PR created automatically by Jules for task 10311574213459934344 started by @edithatogo

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[amazon-q-developer]

This PR successfully adds comprehensive test coverage for pic_contracts.diff_cli.main, achieving the stated goal of 100% coverage. The tests are well-structured and cover both success paths (Markdown and JSON output formats) and error handling (missing files and missing arguments). The test implementation follows pytest best practices with appropriate use of fixtures, assertions, and exception handling. No blocking issues identified.

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